Dialysate solution used in hemodialysis therapy is generally formed by mixing together controlled proportions of liquid dialysate concentrate and water using a "proportioning system." A first type of proportioning system, exemplified by U.S. Pat. Nos. 3,598,727 and 4,172,033 to Willock, 4,107,039 to Lindsay, Jr. et al. and 4,136,708 to Cosentino et al., operates by volumetrically metering liquid concentrate and water to a mixing point using positive displacement piston pumps. The precise ratio of liquid concentrate to water is generally determined by the relative volumes of the pump chambers and in some systems can be varied somewhat by varying the relative travels of the pump pistons.
A second type of proportioning system, exemplified by U.S. Pat. Nos. 4,202,760 to Storey et al., 4,293,409 to Riede et al., 4,508,622 to Polaschegg et al., 3,847,809 to Kopf and 4,082,667 to Seiler, Jr., produces dialysate by introducing dialysate concentrate at a controlled rate into a flow of water. The conductivity of the resulting dialysate is detected downstream from the mix point and is used to regulate the rate at which the liquid concentrate is introduced into the flow of water.
Dialysate proportioning systems are generally included as integral parts of hemodialysis therapy machines, serving to mix the dialysate solution on-line as part of the machine's operation. Preparation of the liquid concentrate used in such systems, however, is an off-line, batch process performed either by the operator of the dialysis machine or by a third party concentrate vendor. In either case, the process is the same. A dry chemical mix of selected salts and other components is dissolved in a large tank of water. Dissolution of the dry chemical mix is effected by a mechanical stirring element or by a recirculation pump that withdraws water from the tank and forcibly reintroduces it, thereby causing turbulence that promotes dissolution. Undissolved chemical that settles to the bottom of the tank, however, tends to collect adjacent the sidewalls where it is not readily dissolved by recirculation or stirrer techniques. Exemplary of the apparatuses used to prepare concentrate is the Renapak Concentrate Manufacturing System marketed by Renal Systems, Inc.
The current practice of preparing dialysate from liquid concentrate suffers from a number of drawbacks. First is the need for complex hydraulic equipment in those proportioning systems that rely on volumetric metering of the components to be mixed. Such complexity increases the cost and reduces the reliability of these systems. Second is the cost, bulk and weight of the liquid concentrate itself. The cost of the concentrate is substantial due to the cost of the concentrate manufacturing equipment and the labor costs associated with operating this equipment. The cost of concentrate purchased from third party vendors is even higher due to the addition of the vendor's profit. The shipping costs are substantial due to the concentrate's volume and weight. Handling costs at the user site are also substantial because the concentrate generally cannot be handled by the regular nursing staff but instead requires use of lift trucks or other such devices.
Accordingly, there remains a need for an improved method and apparatus for preparing dialysate used in hemodialysis therapy.
To fulfill this need, the present invention provides a method and apparatus for creating dialysate solution directly from dry chemicals in a continuous process at the user site, thereby eliminating the intermediate concentrate step and its attendant drawbacks.
The preferred embodiment of the invention includes a drum for containing the dry chemical mix, a conveyor belt, a mixing vessel and monitoring equipment. The drum includes internal baffles that cause the dry chemical to be deposited onto the end of the conveyor belt as the drum rotates. The conveyor belt passes the chemical through a profile-determining gate and into the mixing vessel. The rate at which the belt delivers the dry chemical to the mixing vessel is governed by an electronic circuit that measures the conductivity of the dialysate and adjusts the belt speed as necessary to maintain the dialysate conductivity at a set value.
The preferred mixing vessel is conical in shape, having a wider upper portion and a narrower lower portion. Warm water is introduced in the lower portion of the vessel and flows upwardly to the water surface. As particles of the chemical drop from the conveyor onto the surface, some particles dissolve instantaneously. Other particles, especially larger particles, sink below the surface and gravitate downwardly through the flowing liquid. As a particle sinks to the lower portion of the vessel, it encounters progressively less saline water at a progressively greater flow rate and turbulence. This environment promotes rapid dsssolution of even caked lumps of the chemical. The dissolution process can be speeded further by use of a small stirring device disposed in the mixing vessel.
The dissolved solution (dialysate) flows out the mixing vessel and into a flow controller which regulates the rate at which dialysate is supplied to the downstream dialysate equipment. A restriction in the flow controller serves as a nucleation site for bubble formation. A downstream deaeration pump causes the dialysate to be degassed.
If it is necessary to adjust the pH of the dialysate solution, a small pump, such as a peristaltic pump, a roller pump, or a cylinder type pump, can be used to inject an acidic fluid at a controlled rate into the warm water provided to the mixing vessel. Such a pump can be driven from the same shaft that is used to drive the conveyor belt carrying the dry chemical mix.
Desirably, two spaced apart pairs of electrodes are used to monitor the dialysate concentration and to regulate the conveyor speed (or other control mechanism) appropriately. The first pair of electrodes measures the conductivity in the mixing vessel itself and thus responds to short term variations in the composition of the dialysate solution. The second pair of conductivity probes measures the conductivity of the dialysate downstream from the mixing vessel and is thus useful in maintaining long term regulation of the dialysate solution composition.
These and other features and advantages of the present invention will be more readily apparent with reference to the detailed description, which proceeds with reference to the accompanying drawings.